1. Field of the Invention
This invention relates to a self-emulsifiable polyisocyanate mixture in which hexamethylene diisocyanate (referred to hereinafter as HDI) has been subjected to urethanation, uretdione-forming reaction and isocyanu-rate-forming reaction, an aqueous coating or adhesive composition comprising the same and a water-soluble resin and/or aqueous emulsion, and an aqueous dispersion comprising the self-emulsifiable polyisocyanate mixture and water for use as a paint or adhesive.
2. Description of the Prior Art
Coating compositions or adhesive compositions containing organic solvents have problems of safety and sanitation such as bad influence upon human body, explosive fire or the like, and environmental pollution such as air pollution or the like, so that a development of aqueous system has been actively made. In such an aqueous system, a water-soluble resin or aqueous emulsion has heretofore been used. However, with a one-pack system consisting only thereof, the required physical properties cannot be developed in many cases, and in general a cross-linking agent is co-used for improving weather resistance, adhesiveness and the like. The cross-linking system can rely on various reactions, and among them, examples using a self-emulsifiable polyisocyanate are stated in Japanese Patent Application Kokai No. 62-50,373 and U.S. Pat. No. 4,663,377. In both cases, a self-emulsifiable polyisocyanate into which a nonionic, hydrophilic surfactant has been introduced as a cross-linking agent is used. In particular, it is a known fact that a hydrophilic surfactant-modified, isocyanurate group-containing, self-emulsifiable polyisocyanate is used for the purpose of the heat resistance, weather resistance and adhesiveness of a water-soluble resin and aqueous emulsion.
However, conventional, hydrophilic surfactant-modified, isocyanurate group-containing, self-emulsifiable polyisocyanates have a high viscosity (usually, 2,000 to 3,000 cP/25.degree. C.) and have a great difference in viscosity from water, and from a water-soluble resin having a low viscosity and the like, so that it has been difficult to uniformly disperse the former in the latter. In addition, when the degree of modification with a hydrophilic surfactant is high, the dispersion stability after the emulsification is good, but the viscosity increases at the time of phase transfer of from the water-in-oil type to an oil-in-water type, and hence a strong shearing force is required. Thus, there has been a problem of working. On the other hand, when the degree of modification with a hydrophilic surfactant is low, the affinity of the modified polymer with water is small, and therefore, the viscosity increase at the time of phase transfer is small. Hence, the emulsification can be achieved by a simple stirring, but the dispersion stability after the emulsification is poor and precipitation and the like are caused, so that an addition effect such as enhancement of weather resistance or the like is not developed. Therefore, when only the amount of the hydrophilic surfactant introduced is adjusted, it has been difficult to make the dispersion stability good without adversely affecting the workability.
A more important problem is that even when the dispersion stability of the self-emulsifiable polyisocyanate in a liquid composition is good, the compatibility of the self-emulsifiable polyisocyanate with a polyol resin is inferior, so that in such fields where a good coating appearance is required, such as for paint and the like, it has sometimes been impossible to use the self-emulsifiable polyisocyanate because the gloss and image clarity of the coating are deteriorated though weather resistance and heat resistance are enhanced.